Re: Voltage Regulation

On May 20, 11:49 pm, legg <l...@xxxxxxxxxxxxxxx> wrote:
On Wed, 20 May 2009 17:18:55 -0700 (PDT), James Rollins <james.rolli...@xxxxxxxxx> wrote:
On May 20, 7:00 pm, Phil Hobbs
<pcdhSpamMeSensel...@xxxxxxxxxxxxxxxxxx> wrote:
James Rollins wrote:
http://i41.tinypic.com/scv19i.gif

Derived from a capacitor multiplier with voltage attenuation
capabilities. It is simply several capacitor multipliers in series
with the voltage drop distributed across the bjt's. R4 controls the
load voltage while R1=R2=R3 causes the voltage drops across the
transistors to be evenly distributed.

Theoretically this allows one to use large voltages with the capacitor
voltage decreasing for each stage. The kth capacitor from the bottom
has a voltage of k/n*V while each resistor and transistor has V/n.
Tests show that this has significantly better ripple reduction than
one stage alone for the cases I've tested. A factor of almost 100x.

What seems nice about this circuit is the reduced voltage requirements
of the transistors which also allows for higher beta. The power
dissipation for each is also reduced but the same as using one
transistor. Overall this seems to be a much better circuit than using
one stage and using linear regulation alone.

The real question is how this would work out in practice?

You don't have to do waste voltage like that--just split the base
resistor into N sections and use N capacitors (actually N-1 capacitors,
since you want one at the end for short circuit protection of the
transistor).  You can get 140 dB in one stage in the tens to hundreds of
kilohertz.

I don't know what you are talking about. I don't believe you
understand the circuit. The transistor is used for voltage regulation
and capacitance multiplication. It has to waste voltage so that one
can also attenuate the voltage as it is meant for variable supply.
Even without it one still needs the transistor for capacitor
multiplication.

The capacitor multiplication is used to lower the capacitance
requirements and hence whatever capacitance you are using to get your
140dB I can do the same using a much lower capacitance which
essentially gets multiplied by the beta of the bjt's. With your method
I do not see any need to use resistors and extra capacitors and one
could just use one large filter capacitor. The problem is then how do
you get the voltage attenuation/variable supply?

This used to be a ripple regulator, before the term was misapplied to
hysteritic switching regulators. In any event a capacitive multiplier
is used where regulation is not inherently intended, simply to reduce
ripple actively. The normal circuit simply establishes bias for the
pass element with sufficient headroom to absorb the ripple.

Linear regulation requires a reference and controlled gain. Your
circuit has neither, so it will only regulate as effectively as the
input voltage is regulated. The use of multiple series elementsdefeats
the simplicity of the original without providing the benefit of
regulation.

Gain is a misnomer. Amplifiers to not amplify but attenuate or
modulate. In any case the circuit I gave does have "gain". it is
simply R4/(R1 + R2 + R3 + R4).

The reference is R4 in this case. The capacitors hold the gate current
steady. Just because there is no zener does not mean there isn't a
reference. Of course there are better methods and ultimately I would
be using a precision reference because I would be replacing R4 with
more circuitry that allows for programmability.

In fact the precision regulator in the LM10 data*** is very
similar.

As far as regulation is conserved I believe you are confused. Linear
regulators do no supply power but only attenuate it. Hence why all
linear regulators need some headroom. In fact the circuit I gave is
not much different than your standard linear regulator(See AOE for the
basics of regulation) except a zener is not used for a reference.

I believe you are not looking at the circuit closely enough because it
is a simple regulating circuit that accomplishes all the goals for
basic regulation. What it doesn't have of course are the more advanced
features found in modern regulators such as current limiting, over
voltage, etc. The circuit given of course is not meant to be the
final result but the basis of a practical circuit.


Check out p12 +13 of the LM10 data *** as an example of the basic
concept of a floating adjustible regulator.

http://www.national.com/ds/LM/LM10.pdf

How does the regulator handle the high voltage? The precision
regulator on page 12 still has to handle Vcc across it unless it
cannot adjust the full supply. Hence I would need a regulator that can
handle approximately 1000V. This is quite easy to see since gate of
Q3 is ~ 3 diode drops above the output voltage which needs to swing
approximately the full supply and hence the op amp would need to
handle approximately the full supply. Unfortunately this circuit
won't work. Simulation also



The capacitor being multiplied is C1 in these drawings.

RL

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